The usefulness of ethylene propylene diene terpolymers (EPDM) polymers depends on the cure rate and cure state as well as ease of fabrication. Fabrication describes the collection of polymer properties which allow it to be easily processed by mixing, milling, extrusion or injection. The physical characteristics of cured polymer, i.e., mechanical strength and modulus, will depend on the rate of cure and the nature of the cured state achieved. It is well known that favorable molecular weight distribution (MWD) results in polymers which can have both faster cures and better processing characteristics. The optimum combination of these properties is achieved where the polymers have a particular molecular weight distribution and a particular intermolecular compositional distribution.
A significant amount of effort has been expended by the polymer industry in an attempt to produce such ethylene-propylene polymers with unique molecular weight distributions. Generally, these efforts have been directed toward physical blends of polymers having different MWD, selecting the proper catalyst system or by sequential polymerization in a multiple reactor system. For example, a polymerization is carried out in a first reaction stage to produce a polymer of a given MWD and composition with a subsequent polymerization in a second reactor stage to produce a polymer of a different MWD from that of the first stage and, if desired, of a different monomer composition.
British Pat. No. 1,233,599 is illustrative of this two stage polymerization process. While copolymers of ethylene are disclosed, the examples and disclosure are directed toward polyethylene homopolymers and crystalline copolymers, e.g., 95% ethylene. The preferred catalysts are vanadium compounds such as vanadyl halide, vanadium tetrachloride or vanadium tris-(acetyl-acetonate) in conjunction with an aluminum compound, e.g., Br.sub.2 AlCH.sub.2 Br.sub.2. The different MWDs are obtained by using differing amounts of hydrogen as a chain transfer agent in the first and second stages of polymerization.
U.S. Pat. No. 4,078,131 discloses an ethylene-propylene rubber composition having a bimodal distribution in molecular weights comprising two polymer fractions each having a wide distribution of molecular weights and a monomer composition different from that of the other principal fraction. The polymers are further characterized in that they are formed of: (a) a first principal fraction comprising from about 30% to about 85% (by weight referred to the total weight of elastomers) of molecular weight fractions having an intrinsic viscosity distribution of from about 0.2 to about 3, and an average intrinsic viscosity of about 0.8 to 1.5, an average propylene content between about 36 to about 52% by weight, and a termonomer content of between 0% and about 5%; and of (b) a second fraction comprising about 70% to about 15% of weight of molecular weight fractions having an intrinsic viscosity distribution from about 3 to about 15, and average intrinsic viscosity of about 3.5 to about 7, and average propylene content of between about 26% to about 32% by weight and a termonomer content of about 0 to about 5%. The polymers disclosed have a higher ethylene content in the higher molecular weight fractions of the material.
The polymers are prepared by carrying out polymerization in two separate reactors connected in series. The catalyst systems utilized include organic and inorganic component of a transition metal of Group 4A to 8A of the Mendeleev periodic table of the elements, e.g., VOCl.sub.3, VCl.sub.4, vanadium esters and acetyl aetonates. Co-catalysts include organoaluminum compounds or mixtures of compounds, e.g., aluminum alkyls.
U.S. Pat. No. 3,681,306 discloses a two stage polymerization process for the preparation ethylene-propylene co- and terpolymers. In one embodiment the first stage is a "pipe reactor" and the second stage is a back-mixed pot reactor. The polymerization is carried out so that the average ethylene/alpha olefin ratio in one state is at least 1.3 times the average ratio of the other stage. Any of the coordination catalysts known to be useful in producing EPDM polymers is said to be effective for the process.
U.S. Pat. No. 4,259,468 discloses a broad molecular weight ethylene-propylene-diene rubber prepared using as a catalyst (a) the alcohol reaction product of vanadium oxytrichloride and (b) a mixture of aluminum sesquichloride and ethylaluminum dichloride. The polymer is characterized in that the higher molecular weight fraction contains a larger proportion of the diene than does the lower molecular weight fraction. The polymer has an intrinsic viscosity of about 1.0 to about 6.0 dl/g and a weight average molecular weight/number average molecular weight ratio of about 3 to about 15.
U.S. Pat. No. 4,306,041 discloses a method of manufacture for EPDM type terpolymers which utilizes a two stage polymerization process. Substantially all of the non-conjugated diene monomer is fed to the first stage thereby producing a polymer having a non-uniform diene content.
It has been shown that selecting the appropriate support for titanium based Ziegler catalysts can result in the single stage polymerization of ethylene propylene polymer have a broad molecular weight distribution, e.g., TiCl.sub.4 supported on aluminosilicate; see A. G. Rodinov, et al, Vysokomal, soyed, A23: No. 7, 1560-1567, 1981.
A 1962 study alleges that soluble Ziegler catalysts tend to give EP polymers having uniform monomer distribution as a function of molecular weight. Heterogeneous TiCl.sub.4 -AlEt.sub.3, on the other hand, gives a broad distribution. See G. W. Phillips and W. L. Carrick, "Transition Metal Catalysts, IX. Random Ethylene-Propylene Copolymers with Low Pressure Polymerization Catalysts," J. Am. Chem. Soc., 84, 920-925, 1962.
In the polymerization of ethylene-propylene polymers it has been demonstrated that different catalysts give different EP reactivity ratios. See G. Natla, G. Crespi, A. Valvassori, G. Sartori, Rubber Chemistry and Technology, 36 1608 (1963).
C. Cozewith and G. Ver Strate, Macromolecules, 4, 482 (1971). Cozewith and Ver Strate have determined the R.sub.Et and R.sub.Pr for the system VOCl.sub.3 /EASC to be 10.1 and 0.025 respectively. A co-worker in an unpublished memorandum disclosed that the R.sub.Et and R.sub.Pr for VCl.sub.4 /EASC was found to be 3.91 and 0.224 respectively.